The invention pertains to polyurethanes produced from certain non-linear optical active diols. Such diols display a molecular hyperpolarizability .beta. of the order of 10.sup.-38 m.sup.4 /V. The phenomenon of molecular hyperpolarizability and the related non-linear optical (NLO) effects (Pockel's effect and second harmonic generation) are described in ACS Symposium Series 233, American Chemical Society, Washington, D.C., 1983, and in Angew. Chem. Society, 96 637-51 (1984).
Generally, hyperpolarizable molecules have a delocalized .pi. electron system to which both an electron donor group and an electron acceptor group are coupled directly (D.pi.A system).
Polymers having hyperpolarizable side groups may be subjected to polar orientation in an electric field. As a result, the material also becomes macroscopically hyperpolarizable.
Such a material may be used in an optical switch, in which case an electric circuit is provided on a hyperpolarizable polymer. Such a use is described by J. I. Thackera et al., Appl. Phys. Lett. 52 1031-33 (1988).
The diols useful in the invention are of the formula: ##STR1## wherein D is a trivalent donor group having 2-10 aliphatic carbon atoms and containing at least one alkoxy-oxygen atom or a nitrogen atom forming a tertiary amine, the oxygen or nitrogen atom being linked directly to the benzene ring attached to said donor group, and wherein
R.sub.1 =halogen, R.sub.2, OR.sub.2, COR.sub.2, COOR.sub.2, CN or CF.sub.3 ; PA1 R.sub.2 =H or an alkyl group having 1-3 carbon atoms; PA1 Y=CR.sub.2, C--CN, or N; ##STR2## A=CN, NO.sub.2, CH.dbd.C(CN).sub.2, or CF.sub.3 ; and PA1 n=0-4. PA1 R.sub.1 =halogen, R.sub.2, OR.sub.2, COR.sub.2, COOR.sub.2, CN or CF.sub.3 ; PA1 R.sub.2 =H or an alkyl group having 1-3 carbon atoms; PA1 Y=CR.sub.2, C--CN, or N; PA1 A=CN, NO.sub.2, CH.dbd.C(CN).sub.2, ##STR4## or CF.sub.3 ; and PA1 n=0-4. PA1 R.sub.1 =halogen, R.sub.2, OR.sub.2, COR.sub.2, COOR.sub.2, CN or CF.sub.3 ; PA1 R.sub.2 =H or an alkyl group having 1-3 carbon atoms; PA1 Y=CR.sub.2, C--CN, or N; ##STR6## A=CN, NO.sub.2, CH.dbd.C(CN).sub.2, or CF.sub.3 ; and PA1 n=0-4. PA1 A=NO.sub.2, PA1 R.sub.1 =H, PA1 Y=CH, and ##STR8##
The groups R.sub.1 at the two benzene rings are not necessarily identical. This also applies to the groups R.sub.2 at the double bonds.
The polyurethanes of the current invention are obtained by reacting a D.pi.A diol of the aforementioned type with a diisocyanate.
The essence of the diols useful in the invention is that they have a D.pi.A system. The group at the benzene ring (R.sub.1) is not of vital importance here. Only if there should be a fluorine substituent at the ring to which the acceptor A is also coupled may, on the strength of European Patent Specification No. 0 241 338, a real effect be expected on the macroscopic hyperpolarizability following the incorporation of the diol in a polymer. When selecting substituents at the double bond it should be kept in mind that the D.pi.A system must not be forced out of planarity. If it is sterically forced out of planarity slightly by, for instance, a t-butyl group, then the hyperpolarizability may be influenced negatively.